Some aromatic polycarbodiimides have conventionally been obtained by polymerizing, for example, diphenylmethane diisocyanate (MDI) or tolylene diisocyanate (TDI) as a monomer. Such polycarbodiimides are used as a flame-retardant film or a heat-resistant adhesive because of its excellent heat resistance.
However, the polycarbodiimide obtained from TDI or MDI has a poor solubility and, therefore, may cause gelation or may become solid to be precipitated, with increasing the molecular weight. Thus, a sufficiently high molecular weight polymer cannot be obtained. Further, although the polycarbodiimide film has a heat-resistance in the point that a volatile gas or a decomposed monomer is not generated even if it is exposed to high temperature of 400.degree. C. or more, if it is thermally treated at a temperature of 200.degree. C. or more, it will lose a self-supporting property and become brittle. As a result, the film cannot be practically used. Furthermore, the film has a low moisture resistance at high temperature and under high pressure and also has an insufficient adhesive force when thermally contact-bonded with an adherend, such as a copper foil.